Fine cerium oxide particles are mainly used as catalyst carriers and abrasives for polishing glass, and are required to have largely different characteristics depending on these respective applications.
When used as catalyst carriers, fine cerium oxide particles are required to have a high specific surface area, large pore volume and pores having large diameters, and further required to maintain these values to the utmost extent at high temperature. For example, Japanese Examined Patent Application Publication No. Hei03-24478 discloses cerium(IV) oxide having a specific surface area of 85±5 m2/g or more after calcinated at 350 to 450° C., and preferably 100 to 130 m2/g after calcinated at 400 to 450° C. This oxide is prepared by an aqueous solution of cerium(IV) nitrate being hydrolyzed in nitric acid, the resulting precipitates being separated, washed and optionally dried, followed by calcination at 300 to 600° C.
Japanese Examined Patent Application Publication No. Hei03-24411 discloses cerium(IV) oxide having a specific surface area of 85±5 m2/g or more after calcinated at 350 to 500° C., and preferably 150 to 180 m2/g after calcinated at 400 to 450° C. This oxide is prepared by an aqueous solution of cerium(IV) nitrate being reacted with an aqueous solution containing sulfate ions to precipitate basic cerium(IV) sulfate, and the resulting precipitates being separated, washed and optionally dried, followed by calcination at 300 to 500° C.
Japanese Patent Application Laid-open Publication No. Sho62-275021 discloses an intermediate compound for producing fine cerium oxide such as the above and a method for producing the intermediate compound. The intermediate compound is a cerium(IV) compound represented by a general formula Ce(OH)x(NO3)y.p(cerium oxide).n(H2O) wherein in the formula, x represents a value satisfying x=4−y, y represents a value from 0.35 to 1.5, p represents a value from 0 to 2.0, and n represents a value from 0 to about 20. This method for producing the cerium(IV) compound is composed of hydrolyzing an aqueous solution of cerium(IV) salt with an acid media, separating and optionally heating the resulting precipitates. The shape of this intermediate compound is the same as that of cerium oxide when observed by X-ray diffraction, but the intermediate compound is lost in calcination by 20%. After the intermediate compound is calcinated, cerium oxide having a large specific surface area is obtained.
Cerium oxide powders obtained by the above methods each has a very small crystal particle size of around 5 Å (0.5 nm) when obtained by X-ray diffraction and a large specific surface area of 85±5 m2/g or more, and normally 100 m2/g or more. Sizes of the fine particles are around 0.5 to 2 μm, and the fine particles have fine pores having sizes of around 50 Å.
As an abrasive for polishing glass in a finishing process, cerium oxide is commonly known to be the most effectual and thus widely used. In polishing glass such as a lens, a cerium oxide abrasive obtained by calcinating and grinding bastnasite containing cerium oxide as a main component is generally used. However, such a cerium oxide abrasive used in practice has an average particle size of 1 to 3 μm. In addition, such a cerium oxide abrasive inevitably contains impurities and the content of impurities is uncontrollable because natural ore is used as a starting material. Thus, such a cerium oxide abrasive is unsuitable for use in manufacturing a semiconductor device.
As a method for producing highly pure cerium oxide, there is a method for producing cerium oxide by adding a salt of, for example, carboxylic acid, oxalic acid or acetic acid to an aqueous solution of refined cerium(III) nitrate, cerium(III) chloride or cerium(III) sulfate to precipitate cerium(III) carbonate, cerium(III) oxalate or cerium(III) acetate, filtrating the resulting precipitates, drying the precipitates, and calcinating the precipitates. Cerium(III) oxide is unstable and thus cannot be present in the air; therefore, all of the cerium oxide is present as cerium (quadrivalent) dioxide. In the calcinations in the above producing methods, carboxylic acid, oxalic acid or acetic acid vaporize from the dried precipitates as temperature rises, and cerium oxide is produced. Portions from which carboxylic acid, oxalic acid or acetic acid has been vaporized are present as holes, and thus produced fine particles have very poor crystallinity. Cerium oxide with poor crystallinity has high chemical reactivity and thus causes problems such as burning, “orange peel” and adherence on the surface to be polished when used as an abrasive. Therefore, cerium oxide with poor crystallinity is unsuitable for use in fine polishing, and calcinating temperature needs to be higher. Higher calcinating temperature provides smaller pores and higher crystallinity, while enhancing sintering and producing larger particles. Even if particle sizes are large, fine cerium oxide particles can be obtained by grinding. If a particle size distribution does not need to be considered, it is possible to obtain an average particle size of 0.02 to 2.0 μm by grinding, and thus such grinded cerium oxide may be used in manufacturing a semiconductor device depending on its purpose of use. When fineness of a polished surface is strictly required, particle sizes are required to be similar; however, grinding cannot form fine particles having similar particle sizes.
Patent document 1 proposes, to provide particles having similar sizes, a manufacturing method composed of steps of simultaneously and continuously mixing an aqueous solution of cerium nitrate with an aqueous solution of ammonium so that an equivalent number of ammonium is bigger than that of cerium and pH of a media under reaction is 6 or more, collecting the obtained precipitates by filtration, drying the precipitates, calcinating the precipitates at 600 to 1200° C., and grinding the obtained oxide with a jet mill. Patent document 1 describes that in the case of using cerium(III) nitrate, hydrogen peroxide solution is added so as to change cerium(III) nitrate into cerium(IV) nitrate, and 0.5 to 60% solution of a salt of one or more types of tridentate rare-earth elements selected from a group including lanthanides and yttrium is essentially used. The obtained oxide has an average particle size of 0.5 to 1.7 μm. Thus, this obtained oxide is also unsuitable for use in the case where fineness of a polished surface is strictly required.
As disclosed in Patent document 2, there is a known method for producing a cerium-based abrasive such as the above, the method including steps of mixing a rare-earth element compound containing a rare-earth salt with ammonium hydrogen carbonate in the amount exceeding a stoichiometric ratio for reaction with the rare-earth salt in the water followed by heating, and calcinating the formed and precipitated rare-earth hydroxycarbonate. The above method can produce the cerium-based abrasive which can achieve high polishing speed to some extent, but this cerium-based abrasive is not sufficient as to polishing fineness.
As disclosed in Patent document 3, there is a method for producing a core-shell type monodisperse spherical cerium-polymer hybrid nanoparticle, the method including steps of mixing a cerium salt with polymer in an organic solvent having a high boiling point to obtain a mixture (mixing step) and precipitating cerium oxide by heating the mixture at reflux at 110° C. or higher (heating and refluxing step), and the method characterized by including a step of causing boiling in the heating and refluxing step and a step of rapidly cooling after the heating and refluxing step. This method requires the step of heating and boiling and the step of rapid cooling, and thus complex producing processes and high producing cost are required.
As disclosed in Patent document 4, there is a known cerium oxide compound containing cerium oxide and an element having an ion radius larger than the ion radius of cerium(IV). The cerium oxide compound produced by methods described in Examples 1 and 2 of Patent document 4 can achieve high polishing speed to some extent, but this cerium oxide compound is not sufficient as to polishing fineness.
As disclosed in Patent document 5, there is a known method for producing an intermediate compound of a cerium-based abrasive, the method characterized by including steps of mixing an aqueous solution of at least one type of carbonic acid-based precipitant selected from a group including alkali metal carbonates, alkali metal hydrogen carbonates, ammonium carbonate and ammonium hydrogen carbonate with an aqueous solution of rare-earth compound having CeO2/TREO (total rare earth oxides) of 30% by mass or more so that the carbonic acid-based precipitant is excess as to a stoichiometric amount to thereby form precipitates, and heating the mixture to 60° C. or more without solid-liquid separation. A cerium oxide abrasive produced by the method of Patent document 5 can achieve high polishing speed to some extent, but this cerium oxide abrasive is not sufficient as to polishing fineness.
In a field of ceramic, size and specific surface area of fine particles often correspond with each other by the following equation.Specific surface area (m2/g)=3/rρ
In the equation, r represents diameter (μm) and ρ represents density (g/ml).
In the case of using a material having a large number of pores, the relationship between particle size and specific surface area does not satisfy the above equation. Cerium oxide obtained by the producing methods developed for the case of using cerium oxide as catalyst carriers has a large specific surface area; thus, if corresponding particle size obtained from the above equation is small, namely, 5 nm or less, corresponding particle size is actually around 1 μm.
As for an abrasive, requirements concerning particle size vary according to its applications. When higher fineness of finished surface after polishing is desired, an abrasive is required to be finer particles. For use in manufacturing a semiconductor device, particle sizes are required to be 0.02 to 2.0 μm and also required to be similar. For example, in the case of polishing an insulating interlayer in manufacturing a semiconductor device, a polished surface is required to be fine to have an average surface roughness of around 5 Å (0.5 nm); thus, particle sizes must be 2.0 μm or less to meet the requirement. Meanwhile, smaller particles tend to cause lower polishing speed. Thus, particle sizes of less than 20 nm detract the advantage that cerium oxide provides higher polishing speed compared to colloidal silica. Further, to achieve flatness, particle sizes are required to be as similar as possible. Thus, an average particle size is required to be 0.02 to 2.0 μm, and also particle sizes are required to be similar. Moreover, to achieve flatness, shapes of fine particles are required to be as similar as possible. If each fine particle is formed of a single crystal, respective fine particles have almost the same shapes, and thus highly fine flatness can be achieved.
In polishing silicon oxide such as quartz substrates, it is known that cerium oxide achieves the highest polishing speed. In addition, because an insulating interlayer is composed of silicon oxide, to achieve high speed of polishing an insulating interlayer, cerium oxide is the most suitable. However, as to an insulating interlayer, requirements for flatness and fineness of a polished surface are very strict. At present, only fine particles of colloidal silica have a small particle size distribution and particle sizes of 0.02 to 2.0 μm, and thus colloidal silica is used; however, colloidal silica causes insufficient polishing speed. Hence, cerium oxide having a small particle size distribution and an average particle size of 0.02 to 2.0 μm has been eagerly desired.